The present invention is directed to a braze material, in either paste, paint or tape form, which is useful for repairing gas turbine engine parts and a method for repairing braze joints using said braze material.
Routine maintenance of gas turbine engines involves disassembling the engines, inspecting parts to determine whether they are reusable or require repair or replacement, and ultimately rebuilding the engines with reused, repaired, or replacement parts. Various methods are used to repair gas turbine engine parts including brazing. For example, high pressure compressor stator assemblies, which comprise inner and outer shrouds that support a number of brazed in compressor vanes, are often repaired by debrazing the vanes, cleaning and repairing the shrouds and reusable vanes, and brazing reusable and replacement vanes to the shrouds to rebuild the stator assembly.
The step of cleaning the shrouds to remove oxides that form during engine operation is important to obtain a clean, sound braze when rebuilding a stator assembly. Current cleaning methods include grit blasting and aqueous degreasing. While these methods can satisfactorily clean many shrouds, they typically need either a line of sight to the oxides (for grit blasting) or strong chemical compositions (for aqueous degreasing). Due to the complicated geometry of the shrouds, it is often difficult or impossible to get the line of sight needed for effective grit blasting. The option of using strong chemical compositions is becoming less desirable as repair shops look for ways to lessen the environmental impact of their operations. Even when grit blasting or aqueous degreasing are options, the time and effort required for satisfactory cleaning may be unacceptable to customers. Moreover, grit blasting and/or aqueous degreasing operations can be so aggressive that they damage shrouds to the extent that an entire stator assembly may need to be scrapped. Scrapping a stator assembly is costly to the customer and can delay engine reassembly while a replacement assembly is obtained.
U.S. Pat. No. 5,735,448 relates to a method of repairing surface and near surface defects in superalloy articles such as gas turbine engine components. In this method, after the base metal surface of the article has been cleaned, a repair coating is applied to the base metal surface of the article. The repair coating comprises a composition selected from the group consisting of between about 20 wt % and about 60 wt % volatile organic carrier and between about 20 wt % and about 60 wt % water-based carrier. The repair coating further comprises up to about 8 wt % of a fluxing agent including a halide compound and up to about 5 wt % of a thickening agent. The balance of the coating is metallic filler mix comprising a first finely-divided homogenous particulate component substantially corresponding in composition to that of the article and a second finely-divided homogenous particulate component having as its base the same base metal as that of the article and containing a melting point depressant in a quantity substantially exceeding that present in the article. The volatile organic carrier may be methyl alcohol, ethyl alcohol, or ether. The water-based carrier may be a gel binder such as Nicrobraz Cement-S. The thickening may be any substance capable of congealing the repair coating such as starches, gums, casein, gelatin, and phycocolloids, semisynthetic cellulose derivatives, and polyvinyl alcohol and carboxyvinylates. The fluxing agent includes halide compounds such as fluorides, bromides, chlorides, or mixtures thereof.
The engine repair industry needs a quicker, more reliable method of cleaning gas turbine engine parts required to support brazements.
Accordingly, it is an object of the present invention to provide an improved braze material which may be used to clean and repair engine parts.
It is a further object of the present invention to provide a quicker, more reliable method of cleaning gas turbine engine parts required to support brazements.
It is yet another object of the present invention to provide a more reliable method for repairing gas turbine engine parts.
The foregoing objects are attained by the braze material and the method of the present invention.
In accordance with the present invention, a braze material which cleans and repairs cracks and voids in braze joints in engine parts is provided. The braze material broadly comprises lithium fluoride in an amount sufficient to act as a flux which allows the braze material to flow into a crack or void in a part to be repaired up to an amount where there is no residual lithium or fluoride in the crack or void, and preferably up to about 20% volume, a gel binder in an amount up to about 15% by volume, and the balance comprising at least one of a nickel braze alloy and a cobalt braze alloy. In a most preferred embodiment, lithium fluoride is present in an amount from about 10% to about 15% by volume.
The braze material of the present invention may be applied in a paste form or, alternatively, in a paint or tape form. When the braze material is to be applied in paint form, up to 50% of its volume may comprise a suitable solvent. When the braze material is to be applied in tape form, the gel binder is replaced by a flexible binder of the type usually used to form tape products.
A method for cleaning and repairing a braze joint is also disclosed. The method broadly comprises the steps of forming a braze material containing lithium fluoride in an amount effective to act as a flux which allows the braze material to flow into a braze joint to be repaired up to an amount where there is no residual lithium or fluoride in the crack or void, and preferably up to about 20% by volume, a gel binder in an amount up to about 15% by volume, and the balance comprising at least one of a nickel braze alloy and a cobalt braze alloy, applying the braze material to the braze joint, and heating the braze material to a temperature in the range of from about a temperature sufficient to cause the lithium fluoride to volatize up to about 2300xc2x0 F., preferably up to about 2200xc2x0 F., and most preferably up to about 1950xc2x0 F., for a time period from about 1.0 minute to about 30.0 minutes. As previously mentioned, the braze material may be applied in paste form, in paint form, or in tape form.
Other details of the braze material and the cleaning and repair method of the present invention, as well as other objects and advantages attendant thereto, are set forth in the following detailed description.
It is not uncommon for gas turbine engine repair units to receive engine components, such as high pressure compressor stators, with braze joints that exhibits voids, service induced cracks, and/or lack of braze in joints that are difficult to adequately clean prior to braze repair. It is also known that cleaning methods such as grit blasting and aqueous degreasing may not successfully remove engine oxides. Thus, a braze material that contains a self-cleaning component is highly desirable.
In accordance with the present invention, a self cleaning braze material is provided. The material contains lithium fluoride, a nickel braze alloy, and a gel binder. The lithium fluoride is present in an amount sufficient to act as a flux and enhance the flow of the braze material into the contaminated joint up to an amount where there is no residual lithium or fluoride in the crack or void, and preferably up to about 20% by volume. In a preferred braze material, lithium fluoride is present in an amount from about 10% to about 15% by volume. Lithium fluoride has been found to be a particularly desirable material because it volatizes during the thermal cycle used during the brazing operation. Further, neither the lithium nor the fluoride diffuses into the base material. Instead, the lithium and the fluoride are released to the atmosphere.
Still further, it has been found that the fluoride component in the lithium fluoride bonds with the surface oxides and removes them from the contaminated surface(s). This allows the braze material to properly wet and fill the braze anomaly.
The braze material of the present invention also contains a gel binder to facilitate formation of the braze material into a paste. The gel binder is present in an amount up to about 15% by volume, preferably from about 5 vol % to about 15 vol %. The gel binder may comprise any suitable conventional gel binder known in the art. For example, a gel binder known as Nicrobraz S-Binder manufactured by Wall Colmonoy Corporation may be used. This gel binder is characterized by a specific gravity of 0.998 @ 20xc2x0 C. and a vapor pressure of 17.53 mm Hg @ 20xc2x0 C. and is a non-flammable, non-toxic, odor free, suspension agent that can be mixed with a brazing filler metal powder to form a thixotropic substance.
When the braze material is to be used in paste form, the remainder of the material comprises at least one of a nickel braze alloy and a cobalt braze alloy. The nickel or cobalt braze alloy used in the braze material may comprise any suitable nickel braze alloy and/or any suitable cobalt braze alloy known in the art. For example, a nickel braze alloy consisting essentially of from about 17.5 wt % to about 18.5 wt % nickel and the balance essentially gold may be used in the braze material of the present invention. Alternatively, a nickel braze alloy known as AMS 4777 and consisting essentially of 4.5 wt % silicon, 7.0 wt % chromium, 3.1 wt % boron, 3.0 wt % iron, and the balance essentially nickel may be used in the braze material of the present invention.
In a preferred embodiment of the present invention, the lithium fluoride and the braze alloy are initially each in a powder form where each of the powders has a particle size in the range of from about xe2x88x9260 mesh to about xe2x88x92325 mesh. To form a braze material in paste form, the lithium fluoride powder and the braze alloy powder are mixed together. Thereafter, the gel binder is added to the mixture. The mixture with the gel binder is then stirred gently to form the paste. A syringe may be used to apply the braze material in paste form to fill the crack, void or other braze anomaly to be repaired.
For some applications, it may be desirable to utilize the braze material of the present invention in paint form. To form a paint version, a solvent is added to the mixture. The solvent may comprise any suitable solvent known in the art including but not limited to water-based solvents, alcohol-based solvents, and mixtures thereof. Depending upon the consistency that is required, the solvent may be present in an amount up to about 50 vol %. When the braze material of the present invention is used in paint form, it may be applied to the part or braze joint to be repaired using a brush or spray gun.
For some applications, it may be desirable to utilize the braze material of the present invention in a tape form. To form a tape version, the gel binder is replaced by a flexible binder. The flexible binder may be any conventional binder known in the art used to form a transfer tape. Typically, the flexible binder is present in an amount from about 5% by volume to about 8% by volume.
The braze material of the present invention may be used to repair cracks and voids in gas turbine engine components, particularly those formed from nickel-based and cobalt-based superalloys. For example, the braze material of the present invention may be used to clean and repair braze joints in components formed from Inconel 718 and a material designated as Inconel X-750. Inconel 718 is a metal alloy which consists of 19 wt % chromium, 3.0 wt % molybdenum, 5.1 wt % columbium, 0.90 wt % titanium, 0.50 wt % aluminum, 18 wt % iron, and the balance essentially nickel. Inconel X-750 is a metal alloy which consists of 15.5 wt % chromium, 0.95 wt % columbium, 2.5 wt % titanium, 0.70 wt % aluminum, 7.0 wt % iron, and the balance essentially nickel.
To clean and repair a crack, a void, or another braze anomaly in an engine component, the braze material of the present invention is applied to the crack or the void either as a paste, a paint, or a tape. Thereafter, the braze material and the component being cleaned and repaired are subjected to a thermal treatment cycle wherein the braze material and the component to which it has been applied are heated to a temperature in the range of from about a temperature where the lithium fluoride volatizes to a temperature up to about 2300xc2x0 F., preferably up to about 2200xc2x0 F., and most preferably up to about 1950xc2x0 F., for a time in the range of from about 1.0 to about 30.0 minutes.
During the braze repair of a gas turbine engine component, such as a high pressure compressor stator, a vacuum may be used to prevent oxidation during the thermal treatment and allow adequate braze flow. If the joints have been previously contaminated with oxides from engine service, the vacuum will help prevent the formation of additional oxides. The vacuum however can not sufficiently reduce those oxides present to allow adequate braze flow into the joint. The self-cleaning mechanism of the braze material of the present invention is capable of cleaning the surface of the oxidized and/or contaminated joint during the thermal treatment cycle, allowing the braze material to flow into the joint area.